This invention relates generally to electrical switches and more particularly, it relates to a two-pole make-before-break switch which includes a first contact assembly having a normally-open common contact blade member that is closed prior to closing of another set of normally-open contacts in a second contact assembly so as to complete an electrical connection.
As is generally known, there are a variety of prior art switches which are used to control electrical loads such as motors, lamps, heaters, and any other devices requiring electrical power. The design of a particular switch is often optimized dependent upon the type of load being controlled or switched. When the type of load is inductive, there is typically a very high voltage generated across the switch contacts that first open so as to "break" the current flowing through the inductive load since the current cannot be changed instantaneously. As a result, this high voltage can sometimes cause material to be transferred from one contact to the other contacts thereby creating a very common "pitting" effect on the switch contacts during its use. Therefore, these deposits might impair the effectiveness of the electrical connection between the switch contacts and thus shorten the useful life of the switch.
Another problem encountered with switches that switch high currents is that of low contact force and a subsequent high contact resistance occurring when the switch contacts are first closed. This high resistance may cause the switch contacts to heat up significantly so as to render deformation of material within the switch and sometimes may actually cause the switch contacts to become welded together and thus creating a switch failure and reducing its reliability. This problem is particularly noticeable in the types of switches in which the contact interface is allowed to be "teased" in and out of closure by the operator. For this reason, switches that control high currents are usually designed so as to yield a high contact pressure which is somewhat independent of the force applied to the switch.
In view of the increased public demand for the capability of operating a driver's side window in an automotive vehicle in both a manual lowering mode and an automatic (free run) lowering mode (referred to as "express down" or "one-touch down"), there has arisen a need in the automotive industry for a switch which can implement this feature but yet avoids the problems discussed above. In order to achieve this "automatic-down" feature using solid-state electronics, the switch is required to have one of the set of contacts on the common blade be suspended in a normally-open state.
As a result of this needed feature, the inventors have developed a new switching arrangement which incorporates a first contact assembly having the normally-open common contact blade member that makes contact prior to closing of another set of normally-open contacts by a "snap-action" in a second contact assembly to produce the completed circuit path, thereby maintaining the reliability of the basic mechanical switch. Consequently, the failure on the normally-open common contact blade member has been substantially reduced or eliminated, thereby prolonging the useful life of the switch.